勉略构造带庄科岩片洋壳型变玄武岩锆石U-Pb年龄和Lu-Hf同位素特征:新元古代勉略洋盆存在的直接证据

Zircon U-Pb age and Lu-Hf isotopes of the oceanic metabasalts from Zhuangke tectonic slice,Mianlue tectonic zone:direct evidence of Neoproterozoic Mianlue ocean

庄科岩片位于南秦岭南缘勉略构造带。前人系统的岩石地球化学研究表明,庄科岩片变玄武岩属拉斑玄武岩系列,整体表现出N-MORB特征,为勉略洋盆残余,但一直未获得高精度的测年数据。本文首次获得庄科岩片洋壳型变玄武岩LA-ICP-MS锆石U-Pb年龄为999±4Ma(MSWD=0.08,n=25)、991±6Ma (MSWD=1.50,n=21),具有正的ε_(Hf)(t)值(+11.00-+16.56)和较小的单阶段Hf模式年龄(t_(DM1)为872-1108Ma),符合典型基性岩浆锆石特征,表明庄科岩片变玄武岩的结晶时代为新元古代早期。结合最新区域研究成果认为,勉略洋盆的存在时限为新元古代早中期(约1000-800Ma),并非长期被认为的晚古生代—早中生代。

The Zhuangke tectonic slice crops out in the Mianlue tectonic zone at the southern margin of the South Qinling Orogen. It is widely accepted as a relict of the Mianlue ocean basin based on the tholeiitic and N-MORB(normal mid-ocean ridge basalt)-like geochemical features of its metabasalts. However, the formation age of Zhuangke metabasalts is not well understood due to the lack of high-precision geochronological constraints. Here we report, for the first time, zircon U-Pb ages of 999 ± 4 Ma and 991 ± 6 Ma for the Zhuangke metabasalts obtained by LA-ICP-MS method. Besides, our combined in-situ Lu-Hf isotopic analysis shows that these zircon separates have depleted εHf(t) of +11.00~+16.56 and single-stage Hf mode ages of 872~1108 Ma. These results clearly suggest that the metabasalts from Zhuangke tectonic slice formed in Early Neoproterozoic, and were sourced from a depleted mantle. In the framework of regional geology, we argue that the lifespan of Mianlue Ocean is Early-Middle Neoproterozoic(about 1000 to 800 Ma), rather than the long-held hypothesis of Late Paleozoic-Early Mesozoic.